Induction can be defined as the effect one tissue has on another during development. In this proposal, the interaction between olfactory axons and mitral cells in the olfactory bulb in rat ontogeny will be examined, both in vivo and in organ culutre. It has been suggested, for example, that the axons plan an important role in initiating mitral cell differentiation. In mouse ontogeny, synaptogenesis between olfactory axons and mitral cell dendrites occurs on the 14th day of gestation. On the same day, a specific protein, olfactory marker protein, is demonstrable in receptor cells by an immunocytochemical technique. In rat ontogeny, the first appearance of this marker protein is demonstrable on the 18th fetal day, at about the same time when physiological changes in receptor cells occur, namely an increase in the maximum amplitude of stimulus evoked electro-olfactograms and development of selectivity by receptor cells in their responses to odorant stimuli. Thus, in one animal model the marker protein first appears at the time of synaptogenesis, in the other it appears at the time when receptor cells show signs of physiological maturity. One of the main thrusts of this renewal proposal is to explore these phenomena further in developing rats and in organ cultures of rat tissues to determine whether there are direct relationships among them. The primary olfactory pathway offers the advantage of a relatively simple neural organization and may provide useful basic data for understanding problems of synaptogenesis and synaptic regeneration in the central nervous system.